Perovskite is a well-known type of mixed metal oxides. In general, mixed metal oxides are crystalline compounds and they are classified by general formulas and certain structural-type characteristics of naturally occurring minerals. The perovskite-type metal oxide has the general formula ABO.sub.3 where A and B stand for cations. More than one cation for each A and B may be present. It is at once apparent that there is quite a large number of compounds which fall within the scope of the term perovskite. The compounds and their structure can be identified by X-Ray diffraction.
In prior art, perovskite compounds have been commonly used in the following fields: electrocatalysis, hydrogenation, dehydrogenation and auto-exhaust purification. One drawback with the perovskite-type metal oxides produced in prior art is that, in general, they show a very low BET specific surface area (SS) in the order of 1 m.sup.2 /g. Therefore despite the fact that prior art perovskite-type metal oxides are not expensive to produce, they usually show good catalytic oxidation activities, they are thermally stable and they show a good resistance to poisoning, they have found to date very limited application in place of based-noble metal catalyst used in the field of industrial pollution abatement or automobile emission control. Higher specific surface area perovskite compounds could thus have a great potential as catalyst, particularly in the selective reduction of nitrogen oxide (NO.sub.x) and as electrocatalyst in the cathodic reduction of oxygen.
The known methods for preparing perovskites include sol-gel process, co-precipitation, citrate complexation, pyrolysis, spray-drying and freeze-drying. In these, precursors are prepared by a humid way such as in a mixed gel or in the co-precipitation of metallic ions under the form of hydroxides, cyanides, oxalates, carbonates or citrates. These precursors can thus be submitted to various treatments such as evaporation or combustion (SS.about.1-4 m.sup.2 /g), to the method of explosion (SS&lt;30 m.sup.2 /g), plasma spray-drying (SS.about.10-20 m.sup.2 /g) and freeze-drying (SS.about.10-20 m.sup.2 /g). However, the drawbacks with all of these methods are that either low specific surface area values are reached or that they are complicated and expensive to put into practice.
The most common method for preparing perovskite catalyst is however the traditional method called "ceramic". This method simply consists in mixing constituent powders (oxides, hydroxides or carbonates) and sintering the powder mixture thus formed to high temperature. The problem with this method is that calcination at high temperature (generally above 1000.degree. C.) is necessary to obtain the crystalline perovskite structure. Another drawback is that low specific surface area value is obtained (SS around 1 m.sup.2 /g). An example of such a high temperature heating method is disclosed in U.S. Pat. No. 5,093,301 where a perovskite structure to be used in a catalyst is formed after heating a ground dry powder mixture at 1300.degree. C.
U.S. Pat. No. 4,134,852 issued in 1979 disclosed a variant to the ceramic method by "mechanically alloying", in the old sense of that expression, the constituent powders necessary for the preparation of perovskite catalyst. Indeed, it refers to a conventional grinding in order to obtain a more or less homogenous mixture of particles but not infer any chemical reaction between the components. It can be read in column 7, lines 5-8 of this patent that "[a] mechanically alloyed powder is one in which precursor components have been intimately intradispersed throughout each particle . . . ". Therefore a necessary step of the process disclosed therein to obtain the desired perovskite structure is by heating the "mechanically alloyed" powder composition to an elevated temperature greater than 800.degree. C. (column 7, lines 61-62).
Today, the use of the expression "mechanical alloying" or "mechanosynthesis" refers among other things to a high energy milling process wherein nanostructural particles of the compounds milled are induced. Therefore it also refers to the production of metastable phases, for example high temperature, high pressure or amorphous phases, from crystalline phases stable under ambient temperature and pressure. For example, the structural transformation of alumina (Al.sub.2 O.sub.3), the preparation of ceramic oxides and the preparation of stabilized zirconias by high energy milling or mechanical alloying have already been respectively disclosed in the following references: P. A. Zielinski et al. in J. Mater. Res., 1993, Vol. 8. pp 2985-2992; D. Michel et al., La revue de metallurgie-CIT/Sciences et Genies des materiaux, February 1993; and D. Michel et al., J. Am. Ceram. Soc., 1993, Vol 76, pp 2884-2888. The publication by E. Gaffet et al. in Mat. Trans., JIM, 1995, Vol 36, (1995) pp 198-209) gives an overview of the subject.
However, even if these papers disclosed the use of high energy milling, their authors have only been able to transform their starting product from one phase to another phase. The product resulting from the milling thus still has the same structure. Furthermore, none of them disclose the preparation of perovskite.
There is still presently a need for a simple process, low in cost for producing a valuable perovskite. There is also a need for a perovskite-type metal oxide with a high specific surface area and for a process for producing such a perovskite.